An investigation and evaluation of variable-valve-timing and variable-valve-actuation strategies in a diesel homogeneous charge compression ignition engine using three-dimensional computational fluid dynamics
A three-dimensional computational fluid dynamics modelling was carried out to investigate effects of variable valve timing (VVT) and variable valve actuation (WA) on gas exchange and fuel-air mixing processes in a diesel homogeneous charge compression ignition (HCCI) engine with early fuel injection. Four VVT or WA strategies were conducted for this study: first, a negative valve overlap (NVO) strategy with fixed exhaust-valve-opening (EVO) and intake-valve-closing (IVC) timings but variable valve lifts, referred to as the NVO strategy; second, the NVO strategy with fixed valve profiles but variable EVO and IVC timings, referred to as the EVO strategy; third, the NVO strategy with fixed valve lifts and fixed EVO and IVC timings but variable exhaust-valve-closing (EVC) and intake-valve-opening timings, referred to as the EVC strategy; fourth, WA with just variable valve lifts, referred to as the VMAX strategy. The results indicate that suitable NVO settings will enhance in-cylinder tumble and then increase turbulence intensity before compression end, although the increased NVO has a negative contribution to swirl ratio. It was found that reducing valve lifts alone is not an efficient way to retain the residual gas, but the function of reduced valve lifts will become significantly obvious by combining it with increasing NVO. For the effect of NVO on in-cylinder temperature, longer NVO not only will increase in-cylinder temperature because of the higher residual gas rate but also will improve the in-cylinder temperature homogeneity. On lowering the maximum valve lift or increasing the NVO, the unmixed region of the in-cylinder charge shrinks. The fuel-rich region expands because of the high intake velocity and enhanced turbulence intensity. This is beneficial to the forming of a global homogeneous charge. It has been noted from the current study that, as the droplet distribution may be influenced more by the in-cylinder air motion caused by NVO when the average droplet size is smaller, it is recommended that future studies explore the effects of VVT and WA on diesel HCCI mixing and combustion with various advanced fuel injection strategies.